October 23, 2001 Meeting Notes

 

In attendance:

 

John Carmack

Phil Eaton

Russ Blink

 

 

We spent half of last week at the Space Frontier Conference in LA. We had a good time, and picked up quite a bit of useful information. We will be getting the demo video we had running at the conference converted to mpeg and put on the web site this week.

 

One of the high points was having Gunter Wendt, the NASA pad leader for most of the Mercury, Gemini, and Apollo flights, climb up onto our manned lander and say “Ah, if I was forty years younger, I would want one for Christmas!” He also told us some stories about banging on balky hydrogen peroxide thruster valves to stop them from sputtering, and having a peroxide drum blow the lid through the ceiling due to machining oil from a newly cut drum plug draining down into the drum.

 

Our FAA waiver application for our test range has now been bounced between four different people – the local FSDO, the regional manager, the Fort Worth Center, and finally to the DFW tower. I hope to hear something from them tomorrow.

 

We assembled our new 12 ton floor standing press, which we will be using to press the catalyst packs for the 600 pound thrust motor and the upcoming larger motors. The first pack we made for the 600 pound motor we had one of our neighbors compress in their arbor press, but we need something we can use ourselves during our normal work hours.

 

The 1/4" perforated steel plate at the bottom of the catalyst pack in the big motor is bowing pretty badly. We pressed it back into shape (had to squash something in our new press…), and will be welding some supports underneath it for a temporary fix. We will probably convert from the mild steel, which is rusting badly, to a stainless steel that will have better high temperature strength, but larger motors are almost certainly going to need some extra bracing as well.

 

Our quick attempts at making anti-channel rings last week didn’t turn out so well. The two upper ones that we slit before inserting just pulled away from the edge when they heated up. The one that was folded a bit to make fit seemed to hold its shape ok, but we are going to try to make some this weekend out of thicker metal that needs to be compressed into the bore like a piston ring.

 

Lithium Aluminum Hydride Hybrid Paper

 

Bruno Berger of the SPL was kind enough to dig up a copy of an experimental report on a lithium aluminum hydride ( LiAlH4 ) / peroxide hybrid that I had seen referenced, which is the closest I have found to the interesting-sounding aluminum hydride ( AlH4 ).

 

media.armadilloaerospace.com/misc/LiAl-Hydride.pdf

 

This is not a stellar performer even in theory, but one of the things they wanted to test was regression rates of metalized hybrid grains, which were expected to increase with pressure, which allows throttling with more constant O/F ratios.

 

They were using a reasonable sized cat bed peroxide hybrid engine of around 230 lb thrust, with 200 - 300 psi chamber pressure and an undisclosed nozzle ratio.

 

They pressed fuel grains with 5% polyethylene for increased structural integrity at the expense of a few seconds of Isp, but it was only a 17% difference in grain strength vs pure LiAlH4 , so it's not clear it is necessary to add.  I believe the characteristics of aluminum hydride are similar, so this is a noteworthy point.  The pressed fuel grains gave densities around 94% of theoretical.

 

Theoretical Isp for this mix was calculated as 297 at 1000 psi chamber pressure with 90% peroxide, expanding to 14.7 psi, and 319 vacuum Isp (expansion ratio not listed).

 

They didn't fire at 1000 psi chamber pressure, but calculated the appropriate corrections to conclude that the delivered Isp of 200 at around 250 psi chamber pressure was only 80% of the theoretical Isp, due largely to poor measured C*.

 

They believe that the chamber temperature was not quite high enough to properly burn the aluminum, and discussed possibly adding 10% aluminum powder or 10% magnesium powder to the grain to increase the chamber temperature in hopes of improving combustion efficiency.  Another option they didn't mention, that would probably be higher performing, would be to use 98% peroxide instead of 90% to increase the chamber temperature.

 

LiAlH4 has other disadvantages compared to AlH3, including a low density of 0.917 g/cm3, a low O/F ratio of around 1.6 : 1, and a high regression rate of 0.1 inch / sec at 200 - 268 psi (higher at higher pressures) that would make a more awkward fuel grain size.

 

My conclusions:

 

With 90% peroxide, LiAlH4 is not interesting as a hybrid fuel, because it doesn't perform better than a straight polyethylene fuel grain, which is much cheaper and has a completely gaseous exhaust that is easier on nozzles and generally more benign.

 

If 98% peroxide got the combustion efficiency up, it would offer a moderate Isp advantage over a hydrocarbon, but nothing to get excited about.

 

Aluminum hydride would probably have the same combustion efficiency issues with 90% peroxide, so 98% may be required to get the interesting numbers.

 

New vehicle design

 

We are considering building another manned vehicle before moving on to the high altitude unmanned vehicle.

 

Moving to a dual tank design with a seated pilot in the middle provides twice the propellant, and allows the pilot to be completely enclosed and protected. We can build some experience with strapping a person in before next year’s supersonic vehicle.

 

We can build the new vehicle around the same 2’ diameter engine bulkhead that will be used on the ballistic vehicle. The main engine, manifold, and attitude engines would stay connected together on the bulkhead, while the body tube and tank(s) for either the manned vehicle or the ballistic vehicle are bolted to it. We were going to have to build this anyway, so having it common between two vehicles would be nice.

 

We could use the existing outrigger legs for landing gear, sliding them into mating tubes on the underside of the engine bulkhead. These same mount points could be use for mounting fins for the ballistic vehicle. The existing outriggers are about 14 pounds each, but we could save at least five pounds with minor modifications since they won’t need to support engines or two foot long foam.

 

A seated person can just squeeze into the 2’ filament wound tubes we have. We could cut off a five foot section, which weighs 50 pounds, make a top bulkhead for it, then cut out most of the front third to let people get in. Tall people would have their knees stick out of the front opening a bit. We would hand-lay a fiberglass seat base and foot well, so the pilot is completely separated from the engines. Pad well with energy absorbing rubber foam, mount a full safety harness, and the pilot should be well protected from any orientation of crash.

 

Side or down firing attitude engines? Side firing engines would cut the hover time by 25%, but down firing engines only 10” from the centerline may not have enough control authority.

 

Where to mount the electronics box? Strapped to back? Mounted on top? Sideways on bottom bulkhead, insulated from the engines? Flat on top is easiest, but moves the CG up.

 

High or low side tanks? If mounted high, the plumbing can just make a gentle 90 degree bend directly into the joining T above the manifold. If mounted low, the CG will be better, which will help both tilted landing and the attitude control, but the plumbing would need to turn 270 degrees, potentially leaving a pocket of liquid. There is two feet of choice in vertical mounting position.